Image forming apparatuses (e.g. printers, fax machines, copiers, and multifunction machines having functions of these machines) are known that perform image formation by ejecting liquid such as ink onto a medium with use of, e.g., a liquid ejection device while transporting the sheet. The liquid ejection device comprises a recording head including a liquid ejection head (liquid droplet ejection head) for ejecting droplets of the liquid (recording liquid). It is to be noted that the term “medium” as used herein is hereinafter also referred to as “sheet”, the material of which is not limited to paper. The terms “medium to be recorded on”, “recording medium”, “transfer material”, and “recording sheet”, may be used as synonymous. The terms “recording”, “printing”, and “imaging” may be used as synonymous with the term “image formation”.
The term “image forming apparatus” as used herein indicates an apparatus that forms images by ejecting liquid onto media such as paper, strings, fibers, cloth, leather, metal, plastic, glass, wood, and ceramics. The term “image formation” as used herein indicates not only forming images that have meanings, such as characters and figures, on a medium, but also forming images that do not have meanings, such as patterns, on a medium. The term “liquid” as used herein is not limited to recording liquid and ink, but includes any liquid that can be used for image formation. The term “liquid ejection device” as used herein indicates a device that ejects liquid from a liquid ejection head and is not limited to those for forming images.
There are several types of liquid ejection heads, such as a piezo type and a thermal type. The piezo type head is provided with a diaphragm on the wall of a liquid chamber in which ink is stored. The diaphragm is displaced using a piezo actuator or the like. Then, the volume inside the liquid chamber is changed to increase the pressure, thereby ejecting liquid droplets. The thermal type head is provided with a heating element which generates heat in response to application of a current to a liquid chamber. Bubbles generated due to heat of the heating element increase the pressure inside a liquid chamber, thereby ejecting liquid droplets.
In order to improve the operating speed, image forming apparatuses using such liquid ejection systems are provided with an increased number of nozzles and heads. Recently, line type image forming apparatuses have come into use that can form images using a long head including plural short heads connected together, which allows forming images without scanning with the head.
However, if the length of the head is increased to have more nozzles, the risk of ejection failure increases. One cause of the ejection failure is entry of bubbles into the liquid chamber. The bubbles in the liquid chamber may prevent ink from being fed, resulting in ejection of no ink, or may reduce pressure for ejecting droplets, resulting in poor ejection. Bubbles, even if they are small, near the nozzle, for example, cause ejection of liquid droplets in wrong directions, thereby failing to form an intended image.
The bubbles enter the head in various ways. The bubbles may flow through an ink feed path, or may be introduced from the nozzle. In the case of a head that ejects liquid droplets by boiling an ink film using a heating element, fine bubbles generated during the ejection process can remain in the liquid chamber.
In the case where bubbles enter the liquid chamber, the bubbles are discharged together with ink by carrying out an ejection operation which is not for forming images (often called “idle ejection” or “preliminary ejection”), or by capping a nozzle face for creation of negative pressure to perform a suction operation. Alternatively, the bubbles may be discharged by increasing the pressure of the ink feed path using a pump or the like. When discharging bubbles using these methods, although there are methods to recycle the discharged ink, a large volume of ink is generally used and wasted without being used for image formation. It is to be noted that a recycling method is disclosed in Japanese Patent Laid-Open Publication No. 2005-212350 (Patent Document 1).
Although bubbles in the liquid chamber can be removed by only the above-described methods, bubbles in the ink feed path can be removed by circulating ink inside the ink feed path. This method makes it possible to prevent entry of bubbles from the ink feed path to the liquid chamber without discharging the ink even in the case where a long head is used. However, if the bubbles in the ink feed path are discharged by circulating the ink, a meniscus in the nozzle of the head is broken due to pressure of ink circulation, so that the ink oozes off or the bubbles are introduced from the nozzle.
Japanese Patent Registration No. 2821920 (Patent Document 2) discloses an ink jet recorder configured such that the nozzle face is sealed during ink circulation. This ink jet recorder comprises an ink feed path for guiding ink from an ink tank to a common liquid chamber of a recording head, an ink discharge path for guiding the ink from the common liquid chamber to the ink tank, a discharge port sealing member for sealing the discharge port communicating with the common liquid chamber, and an ink pump for pumping ink from the ink tank to the common liquid chamber. With the discharge port sealed by the discharge port sealing member, the ink is made to circulate by the ink pump from the ink tank through the ink feed path, the common liquid chamber, and the ink discharge path, and back to the ink tank. Thus the air in the ink passage is discharged into the ink tank together with the ink.
Japanese Patent Laid-Open Application No. 08-238772 (Patent Document 3) discloses a head that prevents the pressure of ink circulation from affecting the meniscus. An ink feed path is divided by a partition wall having plural communication passages into a portion near and a portion away from individual liquid chambers of the head. The portion away from the individual liquid chamber is provided with an ink inlet pipe and an ink outlet pipe. More specifically, a circulation path with an ink feed unit therein is provided between a thermal head and an ink tank. A common liquid chamber communicates with plural liquid paths that communicate with plural ink discharge ports for ejecting ink. The common liquid chamber includes a first common liquid chamber and a second common liquid chamber. The first common liquid chamber directly communicates with the liquid paths, while the second common liquid chamber is located at the side of the first common liquid chamber opposite to the side of the liquid chambers and communicates with the first common liquid chamber through the plural communication passages. The second common liquid chamber forms a part of the circulation path, and is provided with an inlet port for the ink flowing from the ink tank and an outlet port to the ink tank.
As mentioned above, if the bubbles in the ink feed path are discharged by circulation of ink, a meniscus in the nozzle of the head is broken due to pressure of the ink circulation, so that the ink oozes off or the bubbles are introduced from the nozzle. To solve such a problem, techniques disclosed in Patent Document 2 and 3 may be used.
However, in the case of the ink jet recorder of Patent Document 2 in which the nozzle face is sealed while circulating liquid, it is difficult to completely seal the nozzle face if a head is long. Further, images cannot be formed by ejecting liquid while circulating the liquid.
In the case of the head of Patent Document 3, the pressure of ink circulation is prevented from affecting the meniscus by dividing the ink feed path with the partition wall having plural communication passages into the portion that is near the individual liquid chambers of the head and the portion that is spaced away from individual liquid chambers and provided with the liquid inlet pipe and liquid outlet pipe so as to prevent bubbles in the ink feed path from entering the individual liquid chamber. However, if bubbles generated in the individual liquid chambers and bubbles introduced from the nozzle are carried upstream due to buoyant forces, the bubbles remain in the portion near the individual liquid chambers because of the presence of the partition wall and cannot be discharged by the circulating current.